1. Field of the Invention
The invention is generally related to antireflective coatings used in microlithography processes and, more particularly, to an antireflective coating with that has light absorbing properties at deep ultraviolet (DUV) wavelengths.
2. Description of the Prior Art
Microlithography is directed to the formation of micron and sub-micron sized patterns on substrates such as semiconductor chips and wafers. Forming patterns of these small dimensions is quite difficult. Often, the surface of the substrate underlying the photoresist will have imperfections and the non-uniformity of the surface, which can sometimes be grainy or have a plurality of undulations, will result in the resist layer having a variable thickness across the surface of the substrate. The uneven topography of the underlying substrate may have variations in height of the same approximate magnitude as the light which is being used to image the photoresist material. Most photoresists are transparent to DUV radiation. Thus, when the photoresist is being patterned, the DUV radiation used to image the photoresist reflects off the surface of the underlying substrate. Silicon and aluminum, which are commonly used in integrated circuit manufacture, are highly reflective to DUV light. The reflection from the surface of the underlying substrate, together with the uneven topography of the underlying substrate, produces an uneven distribution of light in the photoresist material being imaged. This results in a large number of artifacts being produced in the resulting photoresist material.
In order to provide very high definition patterns with micron and submicron sized vias and channels, the number of artifacts produced during photoresist patterning must be minimized. Recent advances in microlithography have demonstrated that including an antireflective coating (ARC) between the photoresist and the substrate can dramatically reduce the number of artifacts in the patterned photoresist. The use of ARCs in microlithographic processes are discussed at length in the prior art. Horn, Solid State Technology, pp.57-62, November, 1991, discloses that resolution better than 0.5 .mu.m using optical lithography is dependent upon two critical processes: ARCs and planarization. The problem of reflective notching of a photoresist material resulting from light reflection from an aluminum component on a silicon substrate is specifically discussed in Horn. U.S. Pat. No. 4,609,614 to Pampalone et al. describes the use of multifunctional acrylates, methacrylate monomers, a dye, and a photoinitiator to produce an absorptive layer for optical lithography. In Pampalone et al., the photoresist layer used for patterning the substrate overlies the absorptive layer. U.S. Pat. No. 4,910,122 to Arnold et al. discloses an ARC interposed under photosensitive layers which includes a light absorbing dye. U.S. Pat. No. 5,126,289 to Ziger which discusses spinning on an ARC of at least three times the thickness of the largest surface irregularity so that the substrate/ARC combination is planar prior to photoresist application. U.S. Pat. No. 5,234,990 to Flaim et al. discloses the use of a polysulfone and polyurea polymers as an ARC.
There is a need for ARC materials which have inherent light absorbing properties provided by monomers in the polymer backbone. Prior art compositions that include dyes dispersed in a polymer carrier have the disadvantage that an extra processing step to evenly distribute the dye throughout the polymer is required, and the disadvantage that small non-uniformities in dye distribution may result in non-uniform anti-reflective properties. In addition, small molecule or monomeric dyes have a tendency to leach out during over coating with the photoresist solution. Because the thickness of the ARC layers should be kept as small as possible, slight non-uniformities in the ARC can adversely affect the patterning results. The polyurea and polysulfone polymers described in U.S. Pat. No. 5,234,990 to Flaim et al. have the advantage of some inherent light absorptive ability provided by the polymer backbone. However, these materials may not be suitable for use in many patterning processes. Thus, it would be advantageous to identify alternative polymers that are useful as ARCs.